This invention relates to antennas, and more particularly to satellites with dual-polarization antennas including a separate feed for each polarization.
Communication satellites are in widespread use for communicating data, video and other forms of information between widely spaced locations on the earth's surface. It is well known that communication satellites are expensive, and that they have a lifetime which is limited by consumption of expendables, notably consumption of propellant which is used for attitude control and for North-South stationkeeping. In order to provide as much propellant as possible at the beginning of a spacecraft's life, the weight of every portion of the spacecraft is scrutinized, and costly tradeoffs are made to save weight to allow on-loading of additional propellant to extend the life of the satellite. The value of a single month of additional operation of a satellite can be millions of dollars, so a weight saving of even a few pounds, for which propellant can be substituted, may result in tens of millions of dollars of savings.
Among the larger structures on the spacecraft are the solar panels, which require a relatively large surface facing the sun in order to intercept sufficient energy to generate electricity for the spacecraft's operation, and the transmitting and receiving antennas.
The antennas are transducers between transmission lines and free space. A general rule in antenna design is that, in order to "focus" the available energy to be transmitted into a narrow beam, a relatively large "aperture" is necessary. The aperture may be provided by a broadside array, a longitudinal array, an actual radiating aperture such as a horn, or by a reflector antenna which, in a receive mode, receives a collimated beam of energy and focuses the energy into a converging beam directed toward a feed antenna, or which, in a transmit mode, focuses the diverging energy from a feed antenna into a collimated beam.
Those skilled in the art know that antennas are reciprocal devices, in which the transmitting and receiving characteristics are equivalent. Generally, antenna operation is referred to in terms of either transmission or reception, with the other mode being understood therefrom.
For various reasons relating to reliability, light weight and cost, many current communication satellites employ "frequency re-use" communications systems. Such a system is described, for example, in U.S. Pat. No. 5,287,543, issued Feb. 15, 1994 in the name of Wolkstein. In a frequency re-use system, independent signals are transmitted from a earth station over a plurality of band limited "channels" which partially overlap in frequency. At the transmitting earth station, mutually adjacent channels are cross-polarized. In this context, cross-polarization means that the signals of a particular channel are transmitted with a particular first polarization, while the signals of the two adjacent channels are transmitted at a second polarization orthogonal to the first. Ordinarily, each of the two orthogonal polarizations are two linear polarizations, which may be referred to as "vertical" and "horizontal", although, as known, precipitation causes rotation of the polarization. In principle, the two orthogonal channels could be right and left circular polarizations, but linear vertical and horizontal are more easily controlled. At the satellite, the vertically and horizontally polarized signals are separated by polarization-sensitive antennas and applied to separate transmission lines. This has the result which, in each channel, tends to suppress the signals relating to the two adjacent channels. Thus, even though the frequencies of the signals in each channel partially overlap, the overlapping frequency adjacent-channel signals are suppressed, which tends to reduce interchannel interference.
In the satellite, the received signals from the vertically and horizontally polarized antennas are converted to a different frequency range, filtered, and amplified as required, to produce independent signals in adjacent channels with partially overlapping frequencies within the converted frequency range, which independent signals are then combined or de-multiplexed, and every other (or alternate) channels are applied to one polarization of a dual polarization antenna for retransmission back to the earth. As in the case of the receving or uplink antenna, the transmitting or downlink antenna tends to maintain a degree of isolation between each channel and its immediate neighbors.
A prior art antenna which has been used for communication satellites includes a first reflector made up of mutually parallel, "vertically" polarized conductors lying along a surface having the shape of a parabola of revolution, and having a focus at which a vertically polarized feed antenna structure is located. Vertically polarized signals are reflected by the first reflector acting as a parabolic reflector, to collimate diverging signals radiated by the feed antenna to form a collimated beam which is directed toward the ground station, and for receiving collimated signals from the ground station and focusing the collimated signals onto the feed antenna. Horizontally-polarized signals, however, pass unimpeded through the vertically polarized conductive elements of the first reflector. A second reflector, located immediately before or immediately after the first reflector, consists of a plurality of mutually parallel, "horizontally" polarized conductive elements, forming a second parabolic reflector having a focal point at a second location different from that of the first focal point. A horizontally polarized feed antenna structure is located at the second focal point.
The above described prior art antenna requires two separate parabolic reflectors, each formed from a elongated conductive grid, and each with a different focal point. The fabrication of the supports which lie between the two reflectors is difficult, and its presence tends to distort the radiation pattern of the rearmost reflector.